The autoimmune pathogenesis of Type 1 diabetes (T1D), the leading childhood autoimmune disease, is experimentally-modeled in the non-obese diabetic (NOD) mouse. NOD mouse studies have revealed that the so-called, diabetogenic, or disease-causing, T cells are central to the pathogenesis of T1D; yet, why these T cells are not effectively controlled via either central or peripheral mechanisms of tolerance is not fully understood. It is clear, however, that these T cells receive critical pro- and anti-proliferative signals from antigen presenting cells (APC) such as dendritic cells (DC), and that these T cell-APC interactions dramatically influence the effector T cell response to pancreatic beta cell antigens and the subsequent course of disease. Yet the molecular mechanisms underlying the control of diabetogenic T cells remain unsolved. Using a diphtheria toxin-mediated ablation model, we found that the myeloid dendritic cells (mDC) subset acts to promote T1D by priming diabetogenic T cells to pancreatic beta cell antigens in vivo. Conversely, depleting plasmacytoid DC (pDC) exacerbates the pathology- increasing both the number and severity of infiltrated islets, suggesting that, once activated, diabetogenic T cells are still under regulatory control by the pDC subset in vivo. Importantly, preliminary studies suggest a direct molecular mechanism, as the presence of intra-islet pDC correlated not only with reduced pathology but also with the localized expression of indoleamine 2,3-dioxygenase (IDO), a potent inhibitor of T cell proliferation. IDO is elicited from pDC by both type 1 and type 2 interferons (IFN). Natural Killer T (NKT) cells regulate diabetogenic CD4+ T cells in an IFN-9-dependent fashion. Using an adoptive transfer model, we found that CD4+ NKT cells are capable of regulating CD4+ diabetogenic effector T cells in vivo. This NKT cell-mediated immunoregulation occurs in the pancreas and pancreatic lymph nodes (PLN) and requires NKT cells to produce IFN-9. The apparent target of IFN-9 is host DC and not the diabetogenic T cells themselves, suggesting that the action of the NKT cells is indirect via conditioning of the host DC compartment. The most likely DC target is the pDC subset; and the most likely molecular effector is the induction of IDO. Preliminary studies suggest a causal link between NKT cells and pDC in the regulation of diabetogenic CD4+ T cells in the NOD mouse. Taken together, these findings have led us to hypothesize: (i) that NKT cells and pDC work in concert to regulate diabetogenic CD4+ T cells and modulate the tempo of insulitis in vivo; (ii) that pancreatic pDC can directly activate NKT cells to produce IFN-9; and (iii) that this IFN-9 induces pDC to in turn make IDO, which results in a localized environment that limits diabetogenic T cell proliferation. To test our hypotheses we propose the following two specific aims: Aim 1: To determine if pDC from the pancreas and PLN of NOD mice directly or indirectly activate NKT cells in vitro and in vivo. Aim 2: To determine if NKT cell-produced INF-9 and pDC-produced IDO establish a regulatory circuit that controls diabetogenic T cells in vivo.